Soon after primary infection, HIV-1 is disseminated in the central nervous system (CNS) where productive replication in brain macrophages and microglia and limited expression of the viral genome in astrocytes may cause an array of toxic events that contribute to HIV-associated neurocognitive disorders (HAND) (Antinori A, et al., Neurology 2007; 69:1789-99). The presence of HIV-1 and expression of viral proteins, even at low levels, in the brains of HIV patients taking combined antiretroviral therapy (cART) is often associated with neurocognitive disorders. It is estimated that greater than 50% of HIV-1 infected individuals with low viral load and high CD4 lymphocyte cell counts, exhibit some form of HAND, and it is suggested that cART may be at least partially responsible for these impairments (Price R W, et al., The Journal of infectious diseases 2008; 197 Suppl 3:S294-306). Based on the severity of disease, HAND is divided into three classes: asymptomatic neurocognitive impairment (ANI), mild cognitive and motor disorders (MCMD), and HIV-1 associated dementia (HAD) (Gannon P, et al., Current opinion in neurology 2011; 24:275-83 and Woods S P, et al., Neuropsychology review 2009; 19:152-68). Some HIV patients however, have no cognitive impairment (NCI). At present, there are no molecular diagnostic biomarkers for different classes of HAND and diagnosis is mostly based on exclusion of other possible causes accompanied by neurological exam, neuropsychological tests, and brain MRI scan. Pathologically, HIV-1 infection usually impacts cortical and subcortical regions and in the case of HIV encephalitis (HIVE) neuronal loss, astrogliosis, infiltrating macrophages, microglial nodules and multinucleated giant cells may be observed (Desplats P, et al., Neurology 2013; 80:1415-23 and Masliah E, et al., Aids 2000; 14:69-74).
At subcellular levels, HIV-1-associated neuronal injury occurs indirectly by a neurotoxic environment created by products of virally infected macrophage/microglia such as chemokines, cytokines, and viral proteins including gp120, Tat and others. Communication between viral proteins and the host, through a variety of signaling receptors including TNFα, NMDA, AMPA and others perturb the homeostasis of neuronal cells leading to their injury and death (Gelman B B, Soukup V M, Schuenke K W, et al. Acquired neuronal channelopathies in HIV-associated dementia. Journal of neuroimmunology 2004; 157:111-9 and Kaul M, et al., Nature 2001; 410:988-94). Endogenous polyamines (putrescine, spermidine, and spermine) are known for modulating NMDA receptor function and early studies demonstrated that HIV-1 Tat-induced neurotoxicity involves the interaction between polyamines and NMDA receptors (Prendergast M A, et al., Brain research 2002; 954:300-7). Besides the effect on neurons, the polyamines, especially spermine, enhance astrocyte coupling through gap junctions (Benedikt J, et al., Neuroreport 2012; 23:1021-5). Importantly, spermine accumulates almost exclusively in glial cells but not in neurons (Laube G, et al., The Journal of comparative neurology 2002; 444:369-86 and Laube G, et al., Glia 1997; 19:171-9). Therefore, these polyamines likely play a central role in astrocyte function.
Intracellular levels of polyamines are tightly regulated by homeostatic interactions between the anabolic and catabolic components of their metabolism. Spermidine/spermine-N1-acetytransferase (SSAT) is the key enzyme in the catabolism of polyamines. It catalyzes the transfer of acetyl groups from acetyl-CoA onto the intracellular polyamines, spermidine or spermine. Acetylation reduces the positive charges on these molecules, alters their binding activity and renders them susceptible to cellular excretion and/or catabolism (Pegg A E, Am J Physiol Endocrinol Metab 2008; 294:E995-1010). Importantly, acetylation also alters their ability to activate homeostatic responses. In mammalian cells, SSAT is tightly regulated and is highly inducible by polyamines (Fogel-Petrovic M, et al., Biochemistry 1996; 35:14436-44). The importance of SSAT in regulating polyamine homeostasis is indicated by its very short half-life, which is on the order of 20 min; this allows the cell to rapidly change enzyme and polyamine levels (McCloskey D E, et al., J Biol Chem 2003; 278:13881-7). SSAT levels can also be induced by a variety of other stimuli including HIV-1 Tat. These increases in SSAT are regulated by translational control mechanisms (Perez-Leal O, et al., Mol Cell Biol 2012; 32:1453-67 and Perez-Leal 0, et al., Amino acids 2012; 42:611-7).
Recently, the ability of SSAT to control polyamine flux through the metabolic pathway was elucidated by showing that the overexpression of SSAT leads to futile metabolic cycling (Kramer D L, et al., J Biol Chem 2008; 283:4241-51). However, the existence of this polyamine cycle and its consequences in patients with HAND has not been determined.
There remains a need for a dependable means of detecting HIV-1-associated neurocognitive disorders, and for monitoring the progress of individuals afflicted with such disorders. The present invention fulfills this need.